Activation error: Signature validation error

"Activation error: Signature validation error." is a known issue that was caused by a PHP upgrade on our license registration server.

If you encounter this Signature Validation error, you must update to the latest version.

Download and install the latest version of PDF Converter, which you can get here: https://www.swingsoftware.com/download/dl-pdfc

You can upgrade your existing installation or perform a fresh install.

Instructions for upgrading can be found here: https://docs.swingsoftware.com/pdf-converter-for-notes/installation-uninstallation/upgrading